Synthesis of Low-Viscosity Ionic Liquids for Application in Dye-Sensitized Solar Cells

Article abstract of DOI:10.1002/asia.201901130

Two types of ionic liquids (ILs), 1-(3-hexenyl)-3-methyl imidazolium iodide and 1-(3-butenyl)-3-methyl imidazolium iodide, are synthesized by introducing an unsaturated bond into the side alkyl chain of the imidazolium cation. These new ionic liquids exhibit high thermal stability and low viscosity (104 cP and 80 cP, respectively). The molecular dynamics simulation shows that the double bond introduced in the alkane chain greatly changes the molecular system space arrangement and diminishes the packing efficiency, leading to low viscosity. The low viscosity of the synthesized ionic liquids would enhance the diffusion of redox couples. This enhancement is detected by fabricating dye-sensitized solar cells (DSSCs) with electrolytes containing the two ILs and I₂. The highest efficiency of DSSCs is 6.85 % for 1-(3-hexenyl)-3-methyl imidazolium iodide and 5.93 % for 1-(3-butenyl)-3-methyl imidazolium iodide electrolyte, which is much higher than that of 5.17 % with the counterpart 1-hexyl-3-methyl imidazolium iodide electrolyte.

Full text of DOI:10.1002/asia.201901130

DOI: 10.1002/asia.201901130
Full Paper
Synthesis of Low-Viscosity Ionic Liquids for Application in
Dye-Sensitized Solar Cells
Yanyan Fang+,[a, b] Pin Ma+,[a, b] Hongbo Cheng,*[c] Guoyu Tan,[d] Jiaxin Wu,[e] Jiaxin Zheng,[d]
Xiaowen Zhou,[a, b] Shibi Fang,[a, b] Yuhua Dai,*[e] and Yuan Lin*[a, b, d]
Abstract: Two types of ionic liquids (ILs), 1-(3-hexenyl)-3-
methyl imidazolium iodide and 1-(3-butenyl)-3-methyl imida-
zolium iodide, are synthesized by introducing an unsaturat-
ed bond into the side alkyl chain of the imidazolium cation.
These new ionic liquids exhibit high thermal stability and
low viscosity (104 cP and 80 cP, respectively). The molecular
dynamics simulation shows that the double bond intro-
duced in the alkane chain greatly changes the molecular
system space arrangement and diminishes the packing effi-
ciency, leading to low viscosity. The low viscosity of the syn-
thesized ionic liquids would enhance the diffusion of redox
couples. This enhancement is detected by fabricating dye-
sensitized solar cells (DSSCs) with electrolytes containing the
two ILs and I2. The highest efficiency of DSSCs is 6.85% for
1-(3-hexenyl)-3-methyl imidazolium iodide and 5.93% for 1-
(3-butenyl)-3-methyl imidazolium iodide electrolyte, which is
much higher than that of 5.17% with the counterpart 1-
hexyl-3-methyl imidazolium iodide electrolyte.
investigated as electrolytes for DSSCs.[3] However, pure imida-
zolium iodide-based ILs have high viscosity and obstruct the
diffusion of the redox couple, limiting the improvement of the
device performance.[4] To decrease the viscosity of IL electro-
lytes, numerous efforts have been made through mixing the
imidazolium iodide ILs with volatile solvents or low-viscosity
ILs.[5] The addition of organic solvents into ILs has an inherent
drawback due to the volatility of organic solvents.[6] The intro-
duction of relatively low-viscosity ILs with anions such as tetra-
cyanoborate, tricyanomethanide and thiocyanate seems to be
promising for high-performance DSSCs.[4,5] However, the appli-
cation of these low-viscosity ILs in DSSCs may be limited by
their potential instability, toxicological and high costs. Hence,
the development of imidazolium iodide-based ILs possessing
low-viscosity is necessary for the enhanced performance of
DSSCs.
Introduction
Because of their simplicity and high flexibility, dye-sensitized
solar cells (DSSCs) have attracted much attention.[1] Room tem-
perature ionic liquids (ILs) are perceived to be an attractive
candidate as a new class of electrolytes,[2] possessing various
advantages such as high thermal stability and ionic conductivi-
ty. In particular, imidazolium salt-based ILs have been widely
[a] Dr. Y. Fang,+ Dr. P. Ma,+ Prof. X. Zhou, Prof. S. Fang, Prof. Y. Lin
Beijing National laboratory for Molecular Sciences
Key Laboratory of Photochemistry
CAS Research/Education Center for Excellence in Molecular Sciences
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: linyuan@iccas.ac.cn
[b] Dr. Y. Fang,+ Dr. P. Ma,+ Prof. X. Zhou, Prof. S. Fang, Prof. Y. Lin
University of Chinese Academy of Sciences
Beijing 100049 (P.R. China)
The viscosity of ILs is determined by the interplay of cou-
lombic and van der Waals interactions as well as hydrogen
bond formation.[7] Decreasing the alkyl chain length of the imi-
dazolium cation reduces the van der Waals interaction be-
tween imidazolium cations, but the electrostatic attraction be-
tween cations and anions increases. Consequently, ILs with a
medium chain length of 3ꢀ6 carbon atoms, such as 1-hexyl-3-
methylimidazolium iodide (HMII), 1-propyl-3-methylimidazoli-
um iodide (PMII) and 1-butyl-3-methylimidazolium iodide
(BMII), have relatively low viscosity, mirroring the behavior of
their conductivity, which is relatively high. Because of the low-
viscosity property, PMII, HMII and BMII have been proven to a
better candidate of choice for IL electrolytes, and they have
achieved higher efficiency in DSSCs.[8] It is believed that if the
viscosity of the iodide melts could be further reduced, the
mass transport and efficiency would be further improved. In
nature, we know that the fluidity of liquid oil with unsaturated
bonds is much more increased than that of fat with side-chain
saturation. The packing efficiency of the oil is diminished by
[c] Dr. H. Cheng
CAS Key Laboratory for Biomedical Effects of Nanomaterials &Nanosafety
National Center for Nanoscience and Technology of China and Institute of
High Energy Physics
Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: chenghongbo025@126.com
[d] G. Tan, Prof. J. Zheng, Prof. Y. Lin
School of Advanced Materials
Peking University
Shenzhen Graduate School
Shenzhen 518055 (P.R. China)
[e] J. Wu, Prof. Y. Dai
Beijing Institute of Petrochemical Technology
Beijing 102617 (P.R. China)
E-mail: daiyuhua@bipt.edu.cn
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/asia.201901130.
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the presence of the unsaturated structure. Learning from that,
we are encouraged to incorporate the unsaturated bond into
high performance HMII and BMII (1-allyl-3-methylimidazolium
iodide with an unsaturated bond from PMII is solid at ambient
temperature), which are hypothesized to have a lower viscosity
than their counterparts with a saturated structure.
Therefore, we synthesized two types of new ILs, 1-(3-hexen-
yl)-3-methyl imidazolium iodide (HeMII) and 1-(3-butenyl)-3-
methyl imidazolium iodide (BeMII) with an unsaturated bond
and introduced into DSSCs for the first time. These two new
ILs possess a lower viscosity (104 cP and 80 cP, respectively, at
278C) than their counterparts with saturated structures HMII
(490 cP) and BMII (400 cP). The low viscosity of the two ILs
could result in the enhanced diffusion of redox couples. Con-
sidering these results, we fabricated DSSCs using the two syn-
thesized ILs separately to assess the potential advantages of
low-viscosity ILs. Using electrolytes with one component IL,
DSSCs with HeMII (BeMII) exhibit higher efficiency up to 6.85%
(5.93%) at 1 sunlight (100 mWcmꢁ2), which is higher than that
of 5.17% with the counterpart HMII electrolyte. To our best
knowledge, the efficiency of 6.85% is one of the best results
for DSSCs with one component ionic liquid electrolyte. Many
strategies, such as mixture of ILs, dye match and so on, are
needed to further improve the efficiency of DSSC based on the
low-viscosity ionic liquids.
Figure 1. (a) TGA curves of the two ILs, (b) J-V curves of DSSCs based on
HeMII electrolytes with different I2 contents, and (c) the relationship be-
tween Jsc and the radiant power for DSSCs using an electrolyte containing
HeMII and I2 with a molar ratio of 20:1.
Results and Discussion
Characterization of HeMII and BeMII
To compare the new ILs with the traditional ILs, we measured
the viscosity of the two new ILs (HeMII, BeMII) and their coun-
terparts with a saturated structure (HMII and BMII). The inher-
ent viscosity of purified HeMII and BeMII was 104 cP and 80 cP,
respectively, at 278C, which is much lower than that of their
counterparts (490 cP for HMII and 400 cP for BMII). This result
proves the feasibility of our design approach by incorporating
the unsaturated bond to achieve low-viscosity ILs.
Figure 2. (a) Snapshots of MD simulation boxes for HMII, (b) snapshots of
MD simulation boxes for HeMII, (c) radial distribution curve of HMII, and
(d) radial distribution curve of HeMII.
The thermal stability of the synthesized HeMII and BeMII
was detected through TGA (Figure 1a). Decomposition of the
two ILs occurs above 2308C, which indicates that the synthe-
sized ILs with an unsaturated structure are thermally stable
and could be a promising candidate electrolyte for practical
DSSCs.
2.8 ꢁ. Nevertheless, HeMII with a rigid double bond shows a
loose system space arrangement with a peak at 4.6 ꢁ, which is
unstable in the system (Figure 2d). The MDS calculation sug-
gests that the double bond introduced in the alkane chain
greatly changes the molecular system space arrangement and
diminishes the packing efficiency of HeMII, which is closely cor-
related with the viscosity of ionic liquids.
Molecular dynamics simulation methodology
The molecular dynamics simulation (MDS) calculation of HMII
and HeMII was performed by applying the LAMMPS package
for investigating the effect of double bonds in alkane chains
on the system space arrangement. Snapshots of the MD simu-
lation boxes and the radial distribution curves for HMII and
HeMII are shown in Figure 2a–d. The packing density of HMII
and HeMII calculated from the simulation boxes (Figure 2a,b)
is 0.1966 and 0.1856 atom per ꢁ, respectively. The MDS calcula-
tion in Figure 2c further reveals that the flexible HMII has a
tight and regular system space arrangement with a peak at
The performance of DSSCs with different I2 contents
The performance of the low-viscosity ILs was first investigated
by adding different contents of iodine. In this part, we chose
compound HeMII for the systematic investigation. The J-V
curves of the champion DSSCs based on HeMII with different
iodine contents are shown in Figure 1b. The J-V curves and
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the corresponding photovoltaic parameters (short circuit cur-
rent density (Jsc), open circuit voltage (Voc), fill factor (FF) and
energy conversion efficiency (h) for the individual device under
different conditions are given in Figure S1, S2 and Table S1.
The photovoltaic performances of these DSSCs were observed
to deteriorate because of the simultaneous decrease in Jsc and
Voc when increasing the I2 concentration. The efficiency de-
clined from 3.27% to 1.76% with the decreasing molar ration
of HeMII and I2from 20:1 to 7:1. The best efficiency is achieved
by adding a small quantity of I2 in HeMII. The optimal content
of I2 in the HeMII electrolyte is much lower than in the refer-
ence report (the molar ration of IL to I2 is usually 10:1).[9] Be-
cause the traditional imidazolium iodide ILs have a relatively
high viscosity, enough iodine is added to guarantee that the
triiodide could be transferred to the counter electrode by dif-
fusion and a Grotthus-type, non-diffusional hopping mecha-
nism.[10] Our photoelectric results indicate that adding a small
quantity of iodine in HeMII could assure the efficient diffusion
of triiodide to the counter electrode because of the higher flu-
idity of HeMII, which is benefit to the mass transport. This can
be proved by the power-law experiment, which refers to the
relationship between Jsc and the radiant power. In Figure 1c, it
can be observed that the Jsc values show a nearly linear re-
sponse to the radiant power, which means that the Jsc is not
limited by the mass transport.[11] Moreover, increasing the con-
centration of I2 mainly enhances recombination and brings se-
rious visible light absorption by I3ꢁ, leading to the worsening
of Voc and Jsc, respectively.[12] By lowering the viscosity of IL, a
small quantity of I2 in the HeMII electrolyte could simultane-
ously guarantee the effective diffusion of redox couples and
delay the recombination reaction, which makes the low-viscos-
ity HeMII potential for application in DSSCs because of the de-
creased corrosion of iodine to electrodes.
Figure 3. (a) J-V curves of the DSSCs based on electrolytes with different LiI
content (20:1:0/1/3/5 represent the molar ratio of HeMII/I2/LiI), (b) steady-
state voltammogram curves of the electrolytes with LiI (20:1:0/1/3/5 repre-
sent the molar ratio of HeMII/I2/LiI), (c) J-V curves of the DSSCs based on
electrolytes with NMBI and GuNCS (20:0:0 represents the molar ratio of
HeMII/NMBI/GuNCS; BeMII or HMII: electrolyte comprises of BeMII or HMII,
I2, LiI, NMBI and GuNCS with a molar ratio of 20:1:3:4.5:4.5.), (d) dark current
of the DSSCs based on electrolytes containing the two additives (the legend
is molar ratio of HeMII/NMBI/GuNCS).
Table 1. The values of Dapp and s.
ꢁ
ꢁ
HeMII/I2/LiI
[molar ratio]
Jlim (I3
)
Dapp (I3
)
Jsc
s
[mAcmꢁ2
]
(10ꢁ6 cmꢁ2 sꢁ1
)
[mAcm-2]
[mScmꢁ1
]
20:1:0
20:1:1
20:1:3
20:1:5
0.581
0.754
1.076
0.734
1.44
1.86
2.66
1.81
6.30
8.89
9.31
9.14
3.69
4.04
6.87
5.39
The effect of the additive content on the performance of
DSSCs
The effect of the LiI concentration on the performance of
DSSCs
Figure 3a shows the J-V curve of the champion DSSCs using
electrolytes with different contents of LiI. The J-V curves and
the corresponding photovoltaic parameters for the individual
device under different conditions are given in Figure S3, S4,
and Table S2. It can be seen that the performance of the
DSSCs was improved when LiI was added by increasing Jsc. For
the electrolyte HeMII containing only I2 and LiI, the DSSCs ach-
ieved an optimal efficiency of 4.47%, which is higher than that
based on HMII (3.43%).
To further improve the performance of DSSCs with the low-vis-
cosity ILs, different amounts of two additives, guanidinium thi-
ocyanate (GuNCS) and N-methylbenzimidazole (NMBI),[14] were
added to the electrolyte. The photovoltaic performance of the
champion DSSCs assembled with additives is depicted in Fig-
ure 3c and the corresponding detailed parameters are sum-
marized in Table 2. The J-V curves and the corresponding pho-
tovoltaic parameters for the individual device under different
conditions are given in Figure S5, S6 and Table S3. The per-
formance of DSSCs is improved when the two additives are
added by increasing Jsc and Voc simultaneously. With the opti-
mal concentration of the two additives, the cell based on the
HeMII electrolyte approaches a maximum value of 6.85%,
which exceeds the 5.17% efficiency of the HMII cell mainly
contributing to the higher Jsc for the HeMII cell. Furthermore,
the photovoltaic parameters (Jsc, Voc, FF and h) of the BeMII-
based cell with the optimal concentration of the two additives
are 11.69 mAcmꢁ2, 675 mV, 0.752 and 5.93% respectively.
The enhancement in Voc is attributed to two factors:[13,14] the
negative shift of the Fermi level of TiO2 due to the negative
ꢁ
To explain the enhancement of Jsc, the Dapp of I3 was esti-
mated from the limiting current (Jlim) of the linear sweep vol-
tammogramsaccording to the literature.[13] The diffusion curves
are presented in Figure 3b. The results of Dapp are listed in
Table 1. It can be observed that the variation trend of Dapp of
ꢁ
I3 agrees with that of Jsc when the molar ratio of LiI to HeMII
is less than 3/20. To further explain the variation trend of Jsc,
the ionic conductivity (s) was investigated. It can be observed
that adding LiI in the electrolytes raises s. Therefore, the im-
ꢁ
proved Dapp of I3 and s are responsible for the enhanced Jsc
after adding LiI.
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cation. These ILs possess a lower viscosity than their counter-
parts with saturated structures. The MDS shows that the
double bond introduced in the alkane chain greatly changes
the molecular system space arrangement. The TGA results indi-
cate that the synthesized ILs with an unsaturated structure are
thermally stable. Due to the low viscosity of the synthesized
ILs, a small quantity of I2 in the electrolyte could simultaneous-
ly guarantee the effective diffusion of redox couples and
retard the recombination reaction, which gives them potential
for application in DSSCs because of the decreased corrosion of
iodine to electrodes. By optimizing the concentration of I2, LiI
and the two additives, DSSCs using electrolytes with a single
component IL HeMII (BeMII) exhibit an efficiency of 6.85%
(5.93%), which is higher than that of 5.17% with the HMII elec-
trolyte. This research may provide a novel perspective for de-
signing low-viscosity ILs for DSSCs and other devices.
Table 2. The champion photoelectric parameters of DSSCs with different
additive contents.
HeMII/NMBI/GuNCS
[molar ratio]
Jsc
Voc
[mV]
FF
h
[mAcmꢁ2
]
[%]
20:0:0
20:1.5:1.5
20:3:3
20:4.5:4.5
20:6:6
BeMII[a]
HMII[b]
9.31
9.11
645
635
685
725
715
675
716
0.744
0.740
0.731
0.700
0.730
0.752
0.744
4.47
4.28
6.27
6.85
5.24
5.93
5.17
12.53
13.52
10.04
11.69
9.71
[a] BeMII or [b] HMII: Electrolyte comprises of BeMII or HMII, I2, LiI, NMBI
and GuNCS with a molar ratio of 20:1:3:4.5:4.5.
surface charge buildup on the surface of TiO2 through adding
NMBI and the restraint of charge recombination in DSSCs due
to the addition of NMBI and GuNCS which is clearly suggested
by the dark current curves shown in Figure 3d. An increase in
Jsc is mainly related to the addition of GuNCS. As previously re-
ported,[14] the adsorbent guanidinium cation on the TiO2 sur-
face is beneficial for the electron injection.
Experimental Section
Materials and reagents
Guanidinium thiocyanate (GuNCS) and iodine (I2) were purchased
from Acros. N-methylbenzoimidazole (NMBI), p-toluenesulfonyl
chloride, 3-buten-ol and 3-hexen-l-ol were obtained from Alfa.
HMII and BMII were prepared according to the literature.[15] FTO
glass (fluorine-doped tin oxide over-layer, and sheet resistance
20 Wcmꢁ2, Hake New Energy Co., Ltd., Harbin) was cut into 1.5ꢂ
0.8 cm2 sheets as a substrate for precipitating TiO2 porous film.
To explore the effect of the additives on the interfacial
charge transfer characteristics for DSSCs, electrochemical impe-
dance spectroscopy (EIS) measurements are performed. The
spectra shown in Figure 4a are measured under under
100 mWcmꢁ2 (AM 1.5) illumination in the open-circuit condi-
tion. The charge transfer resistance Rct2 at the TiO2/dyes/elec-
trolyte interface is obtained by fitting the impedance data em-
ploying the equivalent circuit shown in the inset of Figure 4a,
where Rs is the equivalent series resistance and CPE represents
the chemical capacitance. As Figure 4b shown, the Rct2 values
are decreased with increasing the content of the two additives,
indicating that the charge recombination is suppressed, while
the electrons injection from dyes to the conduction band of
TiO2 is increased. This is attributed to the adsorbent guanidini-
um cation on the TiO2 surface that favorable to increase elec-
tron injection from dyes to TiO2.[14b]
Synthesis route to HeMII and BeMII
The synthesized procedures of HeMII and BeMII are shown in
Scheme 1.
Scheme 1. Synthesis route for HeMII and BeMII.
Procedure for the tosylation of alcohols[16]
Figure 4. (a) Nyquist plots of EIS for DSSCs based on electrolytes with NMBI
and GuNCS (20:0:0 represents the molar ratio of HeMII/NMBI/GuNCS) (the
inset is the equivalent circuit), (b) Rct2 as a function of additive content in
electrolyte.
Pyridine (5 mL) and p-toluenesulfonyl chloride (8.38 g, 44.2 mmol)
were added into 3-a butenol (2.94 g, 40.8 mmol) solution of CH2Cl2
(100 mL) at 08C. The reaction was processed at room temperature
for 24 h. At the end of the processing time, 5% aqueous HCl
(100 mL) was added and the mixture was extracted with CH2Cl2
three times; then, the combined organic layers were dried over an-
hydrous MgSO4 and evaporated. Flash chromatography of the
crude product of 3-butenyl-1-tosylate was performed (yield 87%).
Conclusion
Two novel ILs have been synthesized by introducing an unsa-
turated bond into the side alkyl chain of the imidazolium
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1H NMR (400 MHz, CDCl3): d=7.78 (2H, d), 7.34 (2H, d), 5.60–5.74
(1H, m), 5.04–5.11 (2H, m), 4.03–4.08 (2H, t), 2.44 (s, 3H), 2.35–
2.41 ppm (2H, m).
tions methodology was performed by applying the LAMMPSpack-
age.[18] The Van der Waals’ force was calculated using the 12-6 Len-
nard-Jones famous potentials ULJ(rij) model with a cutoff distance
of 12 ꢁ. All calculations were performed under the periodical
boundary condition in three dimensions. The models of bond,
angle, dihedral, van der Waals and electrostatic interactions were
prepared by using a classical molecular dynamic potential field[19]
to the following equation:
(E)-3-Hexenyl-l-tosylate (yield 90%): obtained from (E)-3-hexen-l-ol;
1
colorless oil: H NMR (400 MHz, CDC13) d=7.78 (2H, d), 7.33 (2H,
d), 5.46–5.50 (1H, m), 5.17–5.20 (1H, m), 4.00–4.02 (2H, t), 2.45
(3H, s), 2.35–2.43 (2H, m), 1.95–2.01 (2H, m), 0.90–0.95 ppm (3H,
t).
X
X
2
2
VðrÞ ¼
kbðr ꢁ r0Þ þ
kqðq ꢁ q0Þ
Procedure for the formation of the iodides
bonds
angles
NaI was added into an acetone solution of 3-butenyl-1-tosylate.
The mixture was heated at reflux overnight, cooled, evaporated,
added to water and extracted with CH2Cl2 three times; then, the
organic layers were dried over anhydrous MgSO4 and evaporated.
Flash chromatography of the crude product of 4-iodo-1-butene as
a yellow oil yielded the following (yield 85%): 1H NMR (400 MHz,
CDC13) d=5.83–5.70 (1H, m), 5.12–5.08 (2H, m), 3.21–3.17 (2H, t),
2.58–2.66 ppm (2H, m).
X
þ
kc ½1 þ cosð n0c ꢁ d0Þꢂ
dihedrals
X
2
þ
kY ðY ꢁ Y0Þ
impropers
ꢄ
ꢂꢀ
ꢁ
ꢀ
ꢁ ꢃ
ꢅ
N
þ Nꢁ1
4eij
12 ꢁ
þ
6
X X
sij
sij
qiqj
rij
rij
rij
i¼1 j¼iþ1
(E)-l-Iodo-3-hexene (yield 85%): obtained from (E)-3-hexenyl-l-tosy-
late: d=5.50–5.56 (1H, m), 5.27–5.33 (1H, m), 3.12–3.16 (2H, t),
2.60–2.67 (2H, m), 2.00–2.07 (2H, m), 0.96–1.01 ppm (3H, t).
All of the symbols shown in the equation represent their original
meaning. All of the parameters used in this work are listed in the
Table S4.[20] The velocity-verlet algorithm was applied to integrate
the equations of motion with a time step equal to 1 fs. First, the
70 ns NPT equilibration was conducted at 500 K. Moreover, the
200 ns NPT equilibration was also operated at 303.15 K to reach
the equilibrium state.
Synthesis of HeMII and BeMII
Briefly, 4-iodo-1-butene or l-iodo-3-hexen (0.3 mol) was added into
the freshly distilled N-methylimidazole (0.25 mol) under Ar to avoid
the oxidation of iodide. After that, the mixture was refluxed for
12 h under Ar. The crude compound was dissolved in water and
extracted with CH2Cl2 at least three times to remove unreacted
iodide. After removing water by the rotary evaporator, the result-
ing liquid was dried in a vacuum at 1008C for 48 h.
Measurements and instruments
The measurements and instruments used in this work are present-
ed in the Supporting Information. Photocurrent density-voltage (J-
V) measurements were performed using a Keithly 2611 Source
Meter (Keithley Instruments, Inc., USA) equipped with a solar simu-
lator (Newport) simulating AM 1.5 sunlight at 100 mWcmꢁ2 irradi-
ance. The ILs were characterized by 1H NMR (Bruker 400m) and
EIS-MS (BIFLEX III). Thermogravimetric analyzer (TGA) measure-
ments were performed using DSC822e by METTLER Toledo Instru-
ments. The apparent diffusion coefficient (Dapp) of the triiodide was
measured by using a Solartron SI 1287 electrochemical instrument,
and the ionic conductivity was measured by using a Solartron SI
1287 electrochemical interface and a Solartron 1255B frequency re-
sponse analyzer according to the literature.[21] Briefly, to investigate
BeMII (yield 50%):1H NMR (300 MHz, CDCl3): d=10.18 (s, 1H), 7.38
(s, 2H), 5.80 (dt, J=17.1, 8.6 Hz, 1H), 5.19–5.03 (m, 2H), 4.45 (t, J=
6.8 Hz, 2H), 4.10 (s, 3H), 2.70 ppm (q, J=6.8 Hz, 2H) (Figure S7).
MS (ESI-MS): positive ion: m/z=137.1 [C8H13N2]+, calculated 137.2;
negative ion: m/z=127 [I]ꢁ, calculated 126.9. FTIR spectra for
BeMII is shown in Figure S8.
1
HeMII (yield 95%): H NMR (300 MHz, CDCl3): d=10.09 (s, 1H), 7.42
(d, J=11.1 Hz, 2H), 5.44 (q, J=17.7, 7.9 Hz, 2H), 4.38 (t, J=6.6 Hz,
2H), 4.10 (s, 3H), 2.67 (q, J=6.3 Hz, 2H), 2.02–1.82 (m, 2H),
0.87 ppm (t, J=7.5 Hz, 3H) (Figure S9). MS (ESI-MS): positive ion:
m/z=165.1 [C10H17N2]+, calculated 165.2; negative ion: m/z=127
[I]ꢁ, calculated 126.9. FTIR spectra for HeMII is shown in Figure S10.
ꢁ
Dapp of I3 ions in electrolytes, symmetric Pt-Pt cells were assem-
bled and characterized by means of the steady-state cyclic voltam-
metry at a scan rate 10 mVsꢁ1, and the voltage was swept from
Assembling of DSSCs
ꢁ
ꢁ0.6 to 0.6 V. Dapp of I3 was calculated from the steady-state cur-
The fabrication procedure for the TiO2 electrode was described in
our previous work.[3a,17] In detail, The TiO2 nanocrystallinefilm con-
sisted of a 2.6 mm transparent TiO2 layer (particle size of 20–30 nm)
and a 3.8 mm scattering layer (a mixture of 250 nm polystyrene
spheres and 20–30 nm TiO2 nanoparticles with a ratio of solid con-
tent of 1:10). After sintering, the films were immersed in a 0.5 mm
N3 dye solution for 24 h. The platinum layered counter electrode
was prepared by a sputtering method. DSSCs were assembled by
injecting the IL electrolyte into the gap between the counter elec-
trode and the dye sensitized TiO2 electrode. The active area of the
cell was 0.20 cm2.
rent (Jlim) using the following equation:
Jlim
d
Dapp
¼
2nFc
where n=2 is the electron number in the electrode reaction, F is
ꢁ
the Faraday constant, and c is the bulk concentration of I3 ions, d
is the thickness of electrolyte between two Pt electrodes.The s
was measured on a self-contained cell model with a sandwich con-
figuration of stainless steel electrode/electrolyte/stainless steel
electrode. The s value was calculated from the measured bulk
electrolyte resistance (R) on the basis of the following equation:
Details for molecular dynamics simulation methodology
s ¼ L=RA
Based on the molecular dynamics simulation of HMII and HeMII,
the effect of double bonds in alkane chains on the system space
arrangement was investigated. The molecular dynamics simula-
where L is the thickness of the electrolyte, A is the contact area be-
tween the electrolyte and stainless steel electrode.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (51303186, 51673204, 21502195) and the
National Materials Genome Project (2016YFB0700600).
Conflict of interest
The authors declare no conflict of interest.
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Keywords: dye-sensitized solar cell · electrolyte · ionic liquid ·
viscosity
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Accepted manuscript online: October 9, 2019
Version of record online: && &&, 0000
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FULL PAPER
Two types of ionic liquids, HeMII and
BeMII, with low viscosity are synthesized
and the best efficiency of dye-sensitized
solar cells (DSSCs) with these ionic liq-
uids is 6.85% and 5.93%, respectively.
Yanyan Fang, Pin Ma, Hongbo Cheng,*
Guoyu Tan, Jiaxin Wu, Jiaxin Zheng,
Xiaowen Zhou, Shibi Fang, Yuhua Dai,*
Yuan Lin*
&& – &&
Synthesis of Low-Viscosity Ionic
Liquids for Application in
Dye-Sensitized Solar Cells
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ